In a conventional radio communications system, a “UL/DL Node Synchronization” function is known as a technique for measuring a transmission delay between a radio network controller and a base station (an uplink transmission delay and downlink transmission delay).
Referring to FIG. 1, the “UL/DL Node Synchronization” function will be explained.
First, as shown in FIG. 1, the radio network controller transmits a DL node synchronization signal to the base station which manages each cell at a transmission time T1, when the base station starts or restarts. Here, the DL node synchronization signal includes information of the above mentioned transmission time T1.
Second, the base station receives the DL node synchronization signal at a reception time T2. In the other words, the DL node synchronization signal transmitted from the radio network controller reaches the base station with a transmission delay (T2−T1).
Third, the base station transmits an UL node synchronization signal including information showing the reception time T2 and a transmission time T3, to the radio network controller at the transmission time T3.
Fourth, the radio network controller measures a transmission delay between the radio network controller and the base station (an uplink transmission delay and downlink transmission delay) in accordance with the times T1, T2, T3 and T4.
Note that the radio network controller and the base station are configured to repeat the above processing at a fixed cycle, in the “UL/DL Node Synchronization” function of the conventional radio communications system.
Generally, in the multicast communications system, whether or not the mobile station can perform the soft combining or the selective combining depends on a transmission timing difference of same data among a plurality of cells.
However, in the conventional radio communications system, the radio network controller is only configured to control the transmission timing, by measuring the transmission delay between the radio network controller and the base station. Therefore, a problem often arises where the mobile station cannot perform the soft combining or the selective combining, since the transmission timing difference of the same data among the plurality of cells increases during the above multicast communications.    (Non-Patent literature 1) 3GPP TS25.402 v5.1.0 Synchronization in UTRAN Stage 2 2002.6
In viewing of the foregoing, it is an object of the present invention to provide a radio communications system and a control apparatus which can improve a reception quality of mobile stations and utilize radio resources effectively, by resolving the problem caused when the conventional radio communications system performs multicast communications.